1. Field of the Invention
This invention relates to a stabilized transmitter used for microwave communication equipment.
2. Description of the Prior Art
A high-frequency transmitter in microwave equipment requires high gain stability against temperature variations.
In general, the high-frequency transmitter comprises an up converter that converts an intermediate-frequency signal into a high-frequency signal and a power amplifier that amplifiers the power of the high-frequency signal. In recent years, power amplifiers using semiconductors (SSPA: Solid-State Power Amplifier) have been popularly used. Such power amplifier comprises a number of semiconductor amplifier elements, and their gains have temperature characteristics as well as variations during manufacturing.
In the prior art, to compensate for temperature-dependent gain variations of the power amplifier, a temperature sensor is provided in the power amplifier, and a variable attenuator is provided in the up converter to achieve gain stability.
On the other hand, since gains of power amplifiers at room temperature vary depending on the manufacturing conditions, a semi-fixed attenuator is provided either on the inside or outside of the up converter, and this semi-fixed attenuator is adjusted such that the total transmission gain of the transmitter becomes a desired gain (see Revised Satellite Communication, P. 166, Corona Publishing Co., 1985; Microwave Circuit for Communications, P. 371, The Institute of Electronics and Communication Engineers, 1981; 1986 Conference of the Institute of Electronics, Information and Communication Engineers, Regime P. 3-264, P. 3-265).
In such a conventional configuration, the aforementioned semi-fixed attenuator must be readjusted to obtain the desired transmission gain when the power amplifier or up converter unit is replaced by a new one due to failure of the up converter or power amplifier unit. In addition, the transmission gain of the transmitter including the up converter, power amplifier, and variable attenuator must be adjusted after being fabricated.